The present invention relates to lift truck vehicles having a powered hydraulic circuit.
In a typical lift truck vehicle, the load handling mechanisms and steering mechanisms are hydraulically operated. Present hydraulic systems incorporated on such lift trucks are powered by a pump or pumps that circulate hydraulic fluid through one or more hydraulic circuits that include a cylinder. An undesirable level of noise can be produced during the operation of these hydraulic systems. This noise is the combined result of three different noise generation mechanisms, typically referred to as noise sources. The first source is airborne noise which directly radiates from the pump. The second source is structureborne noise which occurs when the mechanical vibration of the pump causes other structural components on the lift truck to vibrate and radiate noise. The third source is fluidborne noise caused by pressure pulsations in the hydraulic fluid which are generated when the pump converts low pressure fluid to high pressure fluid. As with structureborne noise, these pressure pulsations cause other structural components, such as a hydraulic line connected to a structural panel, to vibrate and radiate noise. These three sources of noise all originate at the hydraulic pump.
The most common type of hydraulic pump used on lift trucks is a gear pump. External gear pumps are normally used, although internal gear pumps have been utilized. While internal gear pumps are two to three times more expensive and less efficient than external gear pumps, they do generate lower pressure pulsations, and therefore less noise. Because of the drawbacks associated with internal gear pumps, that being greater expense and less efficiency, external gear pumps are the preferred design.
Recent advances in external gear pumps--such as split gears, pressure trapping, and higher quality gears--have been developed with the intent being to lower the magnitude of the pressure pulsations and thereby reduce the amount of noise. Even with the development of advanced external gear designs, forklift truck hydraulic systems still produce an undesirable level of noise.
The absolute minimum noise level that can be achieved on a forklift truck would be that generated as airborne noise by the pump. Totally eliminating the structure and fluidborne noise sources is virtually impossible. However, reducing these generated noises to a level that is substantially below the pump airborne noise level will result in the minimum overall sound pressure for the operator of a lift truck.
Accordingly, the need exists for an apparatus and method for the reduction of structureborne and fluidborne hydraulic circuit noise generated by the hydraulic pump of a forklift truck.